Catalyst and process for the preparation of unsaturated nitriles



United States Patent ABSTRACT OF THE DISCLGSURE Novel catalyst composition comprising a heteropoly acid of molybdenum containing chromium as the central atom, an oxide of arsenic, a supporting carrier, and optionally one of an oxide of chromium, an oxide of manganese, an oxide of iron, and an oxide of boron. The catalyst is useful in the ammoxidative conversion of propylene to acrylonitrile.

This application is a division of our copending application Ser. No. 375,304, filed June 15, 1964, now US. Patout No. 3,293,280.

This invention relates to a process for preparing unsaturated aliphatic nitriles and to novel catalyst compositions useful in the process of the invention. More particularly, it relates to a vapor phase process for the production of acrylonitrile by the reaction of propylene, ammonia and oxygen, in the presence of a catalyst comprising a mixture of an oxide of arsenic alone, or together with an oxide of chromium, manganese, iron or boron and a heteropoly acid of molybdenum containing chromium as the central atom on a carrier.

Unsaturated aliphatic nitriles have utility in a wide variety of commercial applications. Thus, acrylonitrile is known to be useful as an intermediate in organic syntheses of pharmaceuticals, dyes, etc., as well as being the basic component in many synthetic polymers that are useful for preparing fibers, films, molded articles, and the like. It is known that unsaturated aliphatic nitriles can be prepared by reacting olefins with ammonia under oxidizing conditions at elevated temperatures. For example, I. N. Cosby in US. Patent No. 2,481,826, issued Sept. 13, 1949, describes the preparation of lower aliphatic nitriles such as acrylonitrile, methacrylonitrile and acetonitrile by reacting an olefin such as propene, butene-l, etc. with ammonia and oxygen, at 400-600 0., in the presence of various oxidation catalysts and especially vanadium oxides containing molybdenum oxide. Where propene was used as the starting olefin, yields not exceeding about 6 mole percent (Example 6) of acrylonitrile and a substantial amount (10 mole percent) of hydrogen cyanide were obtained. In I D. Idol, IL, US. Patent 'No. 2,904,580, issued Sept. 15, 1959, a vapor phase method is also described for preparing acrylonitrile, wherein a mixture of propylene, ammonia and oxygen is passed over a catalyst comprising the bismuth, tin, and antimony salts of phosphomolybdic and molybdic acids and bismuth phosphotungstate. This process is stated to require not only careful control of the surface area of the catalyst and pressure conditions, but the amount of water employed is a critical factor.

We have now found that by passing a mixture of a short-chain olefin, ammonia and oxygen, in certain proportions, at certain elevated temperatures and in vapor phase, over a catalyst comprising an essentially intimate mixture of 1) an oxide of arsenic alone or together with an oxide of chromium, manganese, iron or boron and (2) a heteropoly acid of molybdenum containing chromium 'ice as the central atom on a carrier the reaction goes smoothly to a relatively higher conversion to the principal product an unsaturated aliphatic nitrile, for example, where the olefin is propylene to acrylonitrile, and to a considerably lesser amount of acetonitrile, with a minimum of by-products as compared with prior art processes such as mentioned above, and that water is not critical to efficient operability of this process since excellent conversions and yields of acrylonitrile can be obtained without water as the diluent. The reaction products can be readily recovered from the efiluent stream from the reactor by conventional means, e.g. by fractional distillation of the effluent condensate.

The catalyst compositions used in carrying out the process of the invention retain their activity and selectivity over relatively long-life periods without appreciable physical deterioration, thereby providing an efficacious vapor phase catalytic process for the production of unsaturated nitriles from certain olefins, ammonia and oxygen, and more especially acrylonitrile from propylene, ammonia and oxygen. The catalyst compositions are especially well adapted for continuous modes of operation as, for example, in a fluidized bed type of reactor.

It is, accordingly, an object of the invention to provide a novel vapor phase process for the preparation of unsaturated aliphatic nitriles, and in particular acrylonitrile from propylene, ammonia and oxygen, by use of a catalyst comprising an essentially intimate mixture of an oxide of arsenic alone or together with an oxide of chrominurn, manganese, iron or boron and a heteropoly acid of molybdenum containing chromium as the central atom on a carrier.

Another object is to carry out the conversion of propylene, ammonia and oxygen to acrylonitrile in a continuous manner.

Another object of the invention is to provide a novel catalyst composition for the preparation of unsaturated aliphatic nitriles, and in particular acrylonitrile from propylene, ammonia and oxygen, comprising an essentially intimate mixture of an oxide of arsenic alone or together with an oxide of chromium, manganese, iron or boron and a heteropoly acid of molybdenum containing chrornium as the central atom on a carrier.

Other objects will become apparent from the general description and examples hereinafter.

In accordance with the invention, we prepare unsaturated aliphatic nitriles, and more especially acrylonitrile, by passing a feed mixture comprising a short-chain olefin containing from 3-4 carbon atoms such as, for example, propylene or isobutylene, ammonia and oxygen, in vapor phase at elevated temperatures, over a catalyst comprising an intimate mixture of (1) an oxide of arsenic alone or together with an oxide of chromium, manganese, iron or boron and (2) a heteropoly acid of molybdenum containing chromium as the central atom on a carrier. The reaction is illustrated with the preferred process, namely, the conversion of propylene, ammonia and oxygen to acrylonitrile, as follows:

A minor proportion of acetonitrile is also formed.

The ratios of the reactants can be varied widely from the theoretical mole ratios of propylenezoxygenzammonia of 121.5:1. Ratios near this, i.e. about 1: about 1.5: about 1 are preferable; however, the process is operable at propylenezoxygen ratios from 120.1 to 1:5, and propylene: ammonia ratios from 1:0.1 to 1:5. Both propylene:oxygen and propylenezammonia ratios may be varied from the theoretical ratios.

Nitrogen may be fed to the reactor. This has no particular effect upon the chemistry involved; but has the practical advantage that since nitrogen is not detrimental, air

may be used as the source of oxygen. In this case, the oxygen to nitrogen ratio is approximately 1:4. Water may be fed to the reactor or it may be omitted. It acts as a diluent and, when used, the preferred amounts in the form of Water vapor range from 0.052.0 moles, or more, per mole of propylene in the feed. The temperature of the reaction can be varied from about 300 C. to about 600 C., but preferably ranges from about 400 C. to about 550 C. The reaction is not significantly pressure dependent. For example, it may be operated satisfactorily at atmospheric pressure, which condition is preferred, but lower or sub-atmospheric pressures and higher or superatmospheric pressures may also be used to give generally similarly good results, e.g., from slightly below atmospheric to about 5 atmospheres. The choice of operating pressures may be governed by economic considerations such as are involved in plant design, product recovery, etc. The gaseous hourly space velocity (GHSV) may also be varied over a wide range, for example, values (STP) as low as 100 may be used, and values as high as 6000 may be used. The preferred space velocity is in the range of about 150 to about 1000. The catalyst may be used either in a fixed bed or in fluidized state. In the latter case, the catalyst exists as small particle which are suspended in an upflowing stream of reactant gases. The latter method of carrying out the invention offers advantages such as, for example, superior temperature control, and less explosive hazard. As previously indicated, water may be included, if desired, although thi is not critical for the reaction goes well without such addition. When isobutylene is substituted for the propylene in the above described process, the principal product is methacrylonitrile.

In general, any type of apparatus that is suitable for carrying out the process of the invention in the vapor phase may be employed, e.g., a tubular type of reactor of furnace which can be operated in continuous or intermittent manner and is equipped to contain the catalyst in intimate contact with the entering feed gases. The reacted gases are then conducted to suitable cooling and separatory equipment and the products further separated and recovered by any of the methods known to those skilled in the art. For example, one such method involves scrubbing the effluent gases from the reactor with cooled water or an appropriate solvent to remove the products of the reaction. In such case, the ultimate nitrile products may be separated by conventional means such as distillation of the resulting liquid mixtures. Unreacted ammonia and olefin may be recovered and recirculated through the system. Spent catalyst may also be reactivated by heating in contact with air.

The fluidized bed reactor employed by us in carrying out the process of the examples consisted of an upright cylindrical tube of Vycor glass of 15 inche length. The lower portion of the tube has an internal diameter of 40 mm. and terminates in a conical bottom which is about 1 inch in length. The lower portion of the tube including the conical bottom is 25 cm. in length. The upper portion of the cylindrical reactor tube has an internal diameter of 55 mm. throughout the greater portion of its length and is tapered at its ends, i.e. where it forms the top end of the cylindrical reactor tube and where it joins the portion of the reactor tube which has an internal diameter of 40 mm. Feed gases were introduced through an inlet tube extending from the top of the reactor and passing through the center of the reactor into the bottom of the catalyst bed. This served to fiuidize the catalyst bed and to preheat the feed gases. The reactor was heated electrically. The eflluent gases and vapors were led from the reactor through a system of traps which were cooled with a Dry Ice bath. Acrylonitrile, water, acetonitrile, traces of hydrogen cyanide, and unreacted ammonia were collected in these traps. The condensate was washed from the thawed traps with water and this product was analyzed by gas chromatography for nitriles and by titration for ammonia.

The stripped gas stream was then led through a gas sampling valve to a wet test meter. Gas streams, were analyzed with Orsat apparatus for unsaturates, carbon dioxide, etc. The stripped gas stream generally contained over percent nitrogen and the rare gases. The unsaturates as determined by Orsat analysis were shown to be propylene by gas chromatography. The balance of the gas stream was propylene, oxygen, carbon dioxide, and carbon monoxide. The techniques described were found expedient and suitable for the practice of this invention. Other suitable techniques, such as are known to those skilled in the art, may be used without in any way limiting the scope of this invention.

In preparing the novel catalyst compositions employed by us in carrying out the process of the invention, an essentially intimate mixture of a heteropoly acid of molybdenum containing chromium as the central atom such as, for example, hexamolybdochromic acid having the empirical formula H CrMo O a carrier and an oxide of arsenic such as arsenic trioxide (AS203) or arsenic pentoxide (As O or mixtures thereof, and also, if desired, an oxide of chromium, manganese, iron or boron or mixtures thereof, is prepared and calcined. The calcination can be carried out, for example, by heating the catalyst mixture at a temperature of from about 200 C. to about 600 C. for a period of several hours or more. The calcined mixture is then reduced to operable granules or particles. Preferably the calcining operation is carried out in the presence of air or other suitable oxygencontaining gaseous mixture. However, it can be conducted in the absence of oxygen.

The heteropoly acid or its ammonium salt can be used in the preparation of the catalyst. Presumably, the ammonium salt of the acid decomposes wholly, or in part, to ammonia and the acid under calcination or during use at reaction temperature. The concentration of the heteropoly acid of molybdenum containing chromium can vary from about 5% to about 60% (preferably 30 to 50%) by weight of the catalyst. The concentration of the oxide of arsenic, calculated as AS205, can vary from about 1% to about 20% (preferably 2 to 9%) by weight of the catalyst. The concentration of the oxide of chromium, manganese or iron can vary from about 0.1% to about 25% (preferably 2 to 10%) by weight of the catalyst. The concentration of the oxide of boron can vary from about 0.1% to a maximum of about 5.0% (preferably 0.5 to 2% calculated as boric oxide, by weight of the catalyst. The heteropoly acid is always present in a greater percent by weight than the oxide of arsenic or the oxide of chromium, manganese, iron or boron. The carrier can comprise about 30% to about 94% by Weight of the catalyst composition. The 'most outstanding results in accordance with the invention are obtained with catalysts comprising 40 to 70% by weight of carrier. The heteropoly acid and the metallic oxides just named are supported on a carrier because it is advantageous to support them on a carrier. The percentages just given are for calcined carrier-supported catalysts. Thus the weight of the catalyst includes the weight of the carrier.

It may be that there is compound formation between the oxide of arsenic and the oxide of chromium, manganese, iron or boron. The chromium, manganese, iron and boron components can be added directly as oxides or in the form of any other compounds, for example, salts such as nitrates, sulfates, etc. which decompose to the oxides on heating. Compounds of boron such as borates, metaborates and pyroborates can also be used. Chromic oxide, rnanganic oxide, ferric oxide and boric oxide are the preferred oxides of chromium, manganese, iron and boron that can be employed in the novel catalyst compositions of the invention which are used in the process of the invention.

Carriers that can be employed, include, for example, silica, silica-alumina, kieselguhr, pumice, titania, zirconia, clay, etc. The use of silica as a carrier is preferred. The

term silica includes silica gel, for example. The catalyst compositions can be readily regenerated by treatment with air or a gas containing molecular oxygen at or above the reaction temperature. While the arsenic-promoted molybdenum heteropoly acid catalysts of the invention are active and selective for the synthesis, the addition of the chromium, manganese or iron component improves the physical strength and activity of the catalyst. The addition of the boron component improves the activity of the catalyst. It is possible that when an oxide of arsenic promoter is used in conjunction with an oxide of chromium, manganese or iron, metal arsenates are formed. Physical strength is important in any solid catalyst, and especially in the case of those to be used in a fluidized state where the catalyst must be strong enough to resist attrition.

The definitions of certain terms used in the examples and Table 1 are defined as follows:

Contact time is the average time in seconds which the reactants spend at reaction conditions in a volume equal to that of the catalyst bed.

Gaseous hourly space velocity (GHSV) is defined as the number of volumes of feed gases (STP) which pass through one volume of catalyst bed in one hour.

The percent conversion to acrylonitrile may be based on propylene or on ammonia.

cined at 200 C. for four hours. The calcined catalyst obtained was tested as described in Example 2.

Example 2 Two hundred milliliters of x 120 catalyst prepared in Example 1 were charged to the fluidized-solids reactor described hereinbefore. Six runs were made with this catalyst using varying conditions. Between runs the catalyst was regenerated by fiuidizing with air for approximately 30 minutes at a temperature of about 430 C. to about 450 C. The runs were of the order of 30 minutes in length. The mole ratios of C H :O :NH :H O:N were 1:1.5: 1:116 for each run. Water was used in vapor form.

The feed stream in milliliters per minute, STP, and the contact time in seconds as well as the gaseous hourly space velocity (GHSV) for runs 1, 2, 3, 4, 5, and 6 are set forth in Table 1 hereinafter.

TABLE 1 Run No. Temp., C H NI-I Air Water Contact GHSV O. Vapor Time The results obtained in runs 1, 2, 3, 4, 5, and 6 are set forth in Table 2 hereinafter.

TABLE 2 Percent Run Convn. Convn. Yield Yield Conv. Yield of No. to ACN, to ACN, of AGN, of ACN, to MeCN, MeCN,

on C311 on NH3 on CBHG on NE; on C H on 03Hfl 1 ACN stands for acrylonitrile. Based on propylene, Example 3 moles acrylonitrile formed X 100 Percent convers1on= moles propylene fed Based on ammonia,

moles acrylonitrile formed moles ammonia fed The yield may be calculated based on propylene or on ammonia.

Based on propylene,

moles acrylonitrile formed total moles propylene consumed Based on ammonia,

moles acrylonitrile formed total moles propylene consumed Conversions and yields to acetonitrile are similarly defined with the moles of acetonitrile formed replacing moles of acrylonitrile in the appropriate expression.

This invention is further illustrated by the following Examples 14 of preferred embodiments thereof although it will be understood that these examples are included primarily for purpose of illustration and are not intended to limit the scope of the invention unless otherwise specifically indicated.

Percent conversion= X 100 Percent yield= X 100 Percent yield= X 100 Example 1 A catalyst comprising by weight 4.7 percent arsenic pentoxide, 39.8 percent hexamolybdochromic acid and 55.5 percent silica was prepared as follows. To 840 g. of 30 percent silica sol were added 222 g. of ammonium hexamolybdochromiate decahydrate. The slurry was stirred and heated and set to a gel. A solution of 21.4 g. of arsenic pentoxide in 125 ml. of water was added to the gel, and the resulting catalyst mixture was stirred and heated. After drying overnight at 120 C., the catalyst was calpentoxide in 150 ml. of water. The slurry was dried, and

then calcined for 2% hours at 200 C. and 1 /2 hours at 500 C. This resulted in a catalyst with high physical strength.

The reactor was charged with 150 ml. of 40 x mesh of the catalyst prepared in Example 3. A feed stream comprising 200 ml. of propylene, 200 ml. of ammonia, 1500 ml. of air, and 200 ml. of water vapor per minute, STP,

was charged to the reactor. The reaction temperature was 475 C. and the contact time was 1.6 seconds. Over 30 minutes of operation, 3.39 g. of acrylonitrile and 0.41 g. of acetonitrile were recovered. This corresponds to an acrylonitrile conversion of 23.9 percent, based on propylene or on ammonia. The yield of acrylonitrile was 41.0 percent based on propylene and 25.5 percent based on ammonia. The conversion to acetonitrile was 3.7 percent.

Example 4 Example 3 was repeated with the single modification that the water vapor used in the feed stream of Example 3 was replaced with nitrogen. Thus, no water vapor was used in this example. The reaction temperature and contact time were the same as in Example 3. Over 30 minutes of operation, 3.81 g. of acrylonitrile and 0.46 g. of

acetonitrile were produced. This corresponds to an acrylonitrile conversion of 26.8 percent, based on propylene or on ammonia. The yield of acrylonitrile was 47.7 percent based on propylene and 28.5 percent based on ammonia. The conversion to acetonitrile was 4.1 percent.

In place of the chromic oxide containing catalyst used in Examples 3 and 4, there may be substituted generally similar catalyst compositions wherein the chroniic oxide is replaced with an oxide of manganese such as for example, Mn O an oxide of iron such as for example, Fe O or an oxide of boron such as boric acid, for example, These catalysts likewise have improved physical properties and give generally similar conversions of propylene to acryloniirile and isobutylene to methacrylonitrile as the chromic oxide containing catalyst.

Other catalyst compositions coming within the specified ranges of components of the invention can also be prepared in accordance with the procedures described hereinbefore. Thus calcined catalytic compositions containing by weight (a) hexamolybdochrornic acid, 2% arsenic pentoxide, 2% chrornic oxide and 86% silica; (b) 25% hexamolybdochromic acid, 8% arsenic pentoxide, 6% chromic oxide and 61% silica; (c) 35% hexamolybdochromic acid, 13% arsenic pentoxide, 10% chromic oxide and 42% silica; (d) 45% hexamolybdochromic acid, 6% arsenic pentoxide, 15% chromic oxide and 34% silica; (e) 50% hexarnolybdochromic acid, 4% arsenic pentoxide and 46% silica; (f) 60% hexamolybdochromic acid, 5% arsenic pentoxide and 35% silica; (g) 40% hexamolybdochromic acid, 5% arsenic pentoxide, 2% manganic oxide and 53% silica; (h) 45% hexamolybdochromic acid, 5% arsenic pentoxide, 10% manganic oxide and 40% silica; (i) 40% hexamolybdochromic acid, 5% arsenic pentoxide, 2% ferric oxide and 53% silica; (j) 35% hexamolybdochromic acid, 4% arsenic pentoxide, 10% ferric oxide and 51% silica; (k) 40% hexamolybdochromic acid, 5% arsenic pentoxide and 55% silica; (l) 30% hexamolybdochromic acid, 6% arsenic pentoxide and 64% silica; (m) 30% hexamolybdochromic acid, 2% arsenic pentoxide, 2% boric acid and 66% silica; (n) 40% hexamolybdochromic acid, 5% arsenic pentoxide, 0.5% boric acid and 54.5% silica; (o) 50% hexamolybdochromic acid, 9% arsenic pentoxide, 1% boric acid and 40% silica and (p) 25% hexarnolybdochromic acid, 5% arsenic pentoxide, 2% boric acid and 68% silica, for example, can be prepared and used in the process of our invention. These catalysts likewise give high conversions of propylene, ammonia, and oxygen to acrylonitrile with a minimum production of acetonitrile and other lay-products when employed in accordance with the process of the invention. Also, as previously indicated, isobutylene may be substituted for the propylene in the example to give satisfactory yields of methacrylonitrile.

This invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be efiected Within the spirit and scope of the invention as described hereinbefore and as defined in the appended claims.

What we claim is:

1. A catalyst composition comprising a calcined mixture of (1) about 5% to about 60% by weight of molybdochromic heteropoly acid, (2) about 1% to about by weight of an oxide of arsenic, calculated as AS205, and (3) about 30% to about 94% by weight carrier, wherein (1) and (2) are supported on said carrier.

2. A catalyst composition as defined in claim 1 wherein said calcined mixture includes a compound selected from the group consisting of an oxide of chromium, an oxide of manganese, an oxide of iron and an oxide of boron, said oxide of chromium, said oxide of manganese and said oxide of iron being present in about 0.1% to about by Weight and said oxide of boron being present in about 0.1% to about 5.0% by weight; and wherein said heteropoly acid is present in greater percent by weight than any metal oxide present in the catalyst.

3. A catalyst composition as defined in claim 1 wherein said molybdochromic heteropoly acid is present in about 30% to about 50% by weight, said oxide of arsenic is present in about 2% to about 9 by weight, and said carrier is present in about 40% to about 70% by weight.

4. A catalyst composition as defined in claim 2 wherein said oxide of chromium, said oxide of manganese, and said oxide of iron are present in about 2% to about 10% by weight, and said oxide of boron is present in about 0.5% to about 2% by weight.

5. A catalyst composition as defined in claim 2 wherein said calcined mixture comprises (1) about 5% to about 60% by weight of a molybdochrornic heteropoly acid, (2) about 1% to about 20% by weight of an oxide of arsenic, calculated as AS205, (3) about 30% to about 93.9% by weight carrier, and (4) about 0.1% to about 25 by weight of chromic oxide, wherein (1), (2) and (4) are supported on said carrier and said heteropoly acid is present in greater percent by weight than any metal oxide present in the catalyst.

6. A catalyst composition as defined in claim 4 wherein said calcinated mixture comprises (1) about 30% to about 50% by weight of a molybdochromic heteropoly acid, (2) about 2% to about 9% by weight of an oxide arsenic, calculated as As O 3) about 40% to about 70% by weight of a carrier, and (4) about 2% to about 10% by weight of an oxide of chromium.

7. A catalyst composition as defined in claim 6 wherein said carrier is silica.

8. A catalyst composition as defined in claim 7 wherein said oxide of arsenic is arsenic pentoxide.

9. A catalyst composition as defined in claim 7 wherein said oxide of arsenic is arsenic trioxide.

10. A catalyst composition as defined in claim 9 wherein said molybdochrornic heteropoly acid is hexamolybdochromic acid.

11. Process for preparing a catalyst composition comprising the steps of:

(1) Adding a member selected from the group consisting of a molybdochromic heteropoly acid and the ammonium salt of a molybdochromic heteropoly acid to a silica sol to form a mixture;

(2) Adding an aqueous solution of an oxide of arsenic to the mixture obtained in (1) (3) Drying the mixture obtained in (2); and

(4) Calcining the dried mixture.

12. Process for preparing a catalyst composition as defined in claim 11 wherein an aqueous solution of a compound which is an oxide or a compound which decomposes to an oxide under the conditions of the process, said compound being selected from the group consisting of a compound of chromium, a compound of manganese, a compound of iron, and a compound of boron, is added to the mixture obtained in (2).

13. Process for preparing a catalyst composition as defined in claim 12 where the molybdochromic heteropoly acid is hexamolybdochromic acid and the step of calcining the dried mixture is carried out at a temperature of from about 200 C. to about 600 C.

References Cited UNITED STATES PATENTS 3,135,783 6/1964 Sennewald et al. 260-4653 3,226,421 12/1965 Giordano et al. 260-4653 3,232,978 2/ 1966 Yasuhara et al. 252456 XR 3,253,014 5/1966 McDaniel et al. 252456 XR 3,254,110 5/1966 Sennewald et al. 252456 XR 3,324,166 6/1967 Sennewald et al. 252455 XR PATRICK P. GARVIN, Primary Examiner.

DANIEL E. WYMAN, Examiner. 

